Method for fabricating large cross section injection molded ceramic shapes

ABSTRACT

A method for fabricating large, greater than one centimeter cross section, high density silicon nitride by injection molding process has been developed. The method requires use of a controlled starting powder, a novel binder removal process, a prefiring step followed by isopressing and conventional sintering.

The Government has rights in this invention pursuant to Contract No. DEN3-168 awarded by NASA/DOE.

CROSS REFERENCE TO RELATED APPLICATION

A co-pending patent application, Ser. No. 716,080, filed concurrentlyherewith, entitled METHOD FOR INJECTION MOLDING AND REMOVING BINDER FROMLARGE CROSS SECTION CERAMIC SHAPES by Bandyopadhyay et al., and assignedto GTE Laboratories Incorporated, assignee of the present application,concerns related subject matter of this application.

FIELD OF THE INVENTION

This invention relates to a method for injection molding of ceramics.More particularly it relates to a method of injection molding highdensity silicon nitride articles having large cross sections.

BACKGROUND

Injection molding of ceramics has been described by several authors inthe open literature (e.g. T. J. Whalen et al., Ceramics for HighPerformance Application-II, Ed. J. J. Burke, E. N. Lenoe, and R. N.Katz, Brook Hill Publishing Co. 1978, pp. 179-189, J. A. Mangels,Ceramics for High Performance Application-II, pp. 113-129, G. D.Schnittgrund, SAMPE Quarterly, p. 8-12, July 1981, etc.) and in patentliterature (e.g. M. A. Strivens, U.S. Pat. No. 2939199, 1960, I. A.Crossley et al., U.S. Pat. No. 3882210, 1975, R. W. Ohnsorg, U.S. Pat.No. 4144207, 1979, etc.). Several papers have also been published by GTEauthors (e.g. C. L. Quackenbush et al., Contractors Coordination MeetingProceedings, 1981, G. Bandyopadhyay et al., Contractors CoordinationMeeting Proceedings, 1983). The general process routing for injectionmolding is well known. It includes (a) compounding which involves mixingthe high surface area ceramic with molten organic binder, (b) injectionmolding by which the powder/binder mix is formed into a given shape in ametallic mold, (c) binder removal which must be accomplished withoutdisrupting the ceramic structure, and (d) consolidation of the part bysintering and/or by hot isostatic pressing. Significant effort has beenmade by various researchers to determine the effects of starting powderparticle size and size distribution on moldability of powder, and toidentify binder systems that allow easy compounding, molding and binderburnout (without disruption) from the part. Although volumes of patentliterature now exist on powder requirements, different binder conceptsand binder removal processes, it is generally recognized that injectionmolding and binder removal from a large, complex cross section part(e.g. rotors for turbine engines) poses a very difficult task because ofinternal and external cracking during burnout. An extensive evaluationof the patent literature reveals that in most cases only small crosssection (less than 1.00 cm) parts were considered as examples, or thatthe cross sections and complexities were not revealed. None of thesereferences described fabrication of large, complex cross section siliconnitride parts, specifically fabricated by injection molding andsintering. GTE Laboratories has developed a process routing which ishighly successful for fabrication of good quality small cross sectioninjection molded and sintered parts, such as turbine blades and vanes,in large quantities. Since this development, improvements in bindercomposition (K. French et al., U.S. Pat. No. 4,456,713) allowed furthersimplification of molding and binder removal procedures. This processrouting however, failed to produce an externally crack-free injectionmolded ceramic article having a large cross section one centimeter orgreater (e.g, turbine rotor and turbocharger sized test parts). It hasbeen established that the use of a submicron starting powder, such assilicon nitride containing Y₂ O₃ and Al₂ O₃, and a binder results incertain fundamental difficulties which causes external and internalcracking in large cross section parts.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide a newand improved method of injection molding, binder removal and densifyinga ceramic article having a large cross section which is crack-free. Aprocess routing capable of making large, complex cross section parts canalways be used for fabrication of small parts. The reverse may notalways be true.

SUMMARY OF THE INVENTION

This and other objects, advantages, and capabilities are provided inaccordance with one aspect of the present invention wherein there isprovided a method for fabricating a large cross-sectional ceramicarticle by injection molding comprising the following steps:

Step 1. A ceramic powder having a mean particle size from about 2 toabout 12 microns is compounded with about 34 to about 42 v/o of abinder.

Step 2. The product from step 1 is injection molded to form a moldedceramic article having a cross section greater than one centimeter.

Step 3. The product from step 2 is embedded in a setter bed containing asetter powder.

Step 4. A binder retarding layer of the setter powder is formed on themolded ceramic article by heating the product of step 3 in anon-oxidizing environment at a heating rate equal to or greater than 1°C. per hour to 450° C. The setter powder retards the removal of thebinder from the molded ceramic article sufficiently to maintain equal toor greater than 80 w/o of the binder within the molded ceramic articleuntil 400° C. is obtained.

Step 5. Between 400° and 450° C. the binder is removed primarily byvaporization from the part surface by increasing the temperature fromstep 4 to 450° C. at a rate equal to or greater than 1° C. per hour in anon-oxidizing environment.

Step 6. The heating of the product of step 5 is increased to 600° C. andthe product is maintained at that temperature in air for a periodsufficient to completely remove the binder from the molded ceramicarticle.

Step 7. The product from step 6 is cooled to room temperature to obtainan externally crack-free injection molded and binder free ceramicarticle having a cross section greater than one centimeter.

Step 8. The product from step 7 is isopressed at a pressure equal to orgreater than 5,000 psi.

Step 9. The product from step 8 is sintered at a temperature sufficientto obtain a density greater than 98% of theoretical density.

Another aspect of the present invention wherein another method isprovided for fabricating a large cross-sectional ceramic article byinjection molding comprising the following steps:

Step 1. A ceramic powder having a mean particle size from about 2 toabout 12 microns is compounded with about 34 to about 42 v/o of abinder.

Step 2. The product from step 1 is injection molded to form a moldedceramic article having a cross section greater than one centimeter.

Step 3. The product from step 2 is embedded in a setter bed containing asetter powder.

Step 4. A binder retarding layer of the setter powder is formed on themolded ceramic article by heating the product of step 3 in anon-oxidizing environment at a heating rate equal to or greater than 1°C. per hour to 450° C. The setter powder retards the removal of thebinder from the molded ceramic article sufficiently to maintain equal toor greater than 80 w/o of the binder within the molded ceramic articleuntil 400° C. is obtained.

Step 5. Between 400° and 450° C. the binder is removed primarily byvaporization from the part surface by increasing the temperature fromstep 4 to 450° C. at a rate equal to or greater than 1° C. per hour in anon-oxidizing environment.

Step 6. The heating of the product of step 5 is increased to 600° C. andthe product is maintained at that temperature in air for a periodsufficient to completely remove the binder from the molded ceramicarticle.

Step 7. The product from step 6 is cooled to room temperature to obtainan externally crack-free injection molded and binder free ceramicarticle having a cross section greater than one centimeter.

Step 8. The product from 7 is heated to a temperature less than or equalto about 1500° C. in nitrogen atmosphere.

Step 9. The product from step 8 is isopressed at a pressure equal to orgreater than 5,000 psi.

Step 10. The product from step 9 is sintered at a temperature sufficientto obtain a density greater than 98% of theoretical density.

DETAILED DESCRIPTION OF THE INVENTION

It has been established that the use of a submicron starting powder(such as processed silicon nitride containing Y₂ O₃ and Al₂ O₃) and awax based binder (both of which are used for small cross section parts)resulted in certain fundamental difficulties which caused external andinternal cracking in large cross section parts. The cracking problemincreases with increasing part size and part complexity. The fundamentaldifficulties arise due to migration of liquid binder driven by capillaryaction from the interior of the part to its surface. The liquid binderoften carries fine submicron silicon nitride particles from the bulk tothe surface causing a density gradient, shrinkage gradient and cracking.In addition to fine particle migration, the capillary forces exert anoutward drag on all particles causing them to start compaction at thepart surface. As a result, the surface becomes rigid and prevents partshrinkage. Thus as the binder loss continues, the interior shrinks awayfrom the rigid surface region causing the formation of cracks. Thisunderstanding of binder loss and cracking mechanisms is the basis forthe present invention. The concepts pursued in this invention are asfollows:

Reduce the amount of fine particles (less than one micron siliconnitride particles) or eliminate fine particles from the starting powderto reduce or eliminate their preferential migration to the surface.

Increase the particle size of the starting powder to reduce thecapillary forces which, in turn, reduce the outward particle drag.

Develop a burnout setter powder composition and a thermal cycle, whichallows liquid migration at the highest possible temperatures so that theliquid viscosity is at its lowest level when removed from the part.

Optimize powder morphology, powder volume loading, compounding, molding,and burnout to fabricate externally crack-free burned out large crosssection part. The part at this stage may contain internal defects.

Eliminate internal defects by applying compressive stresses on apartially or a fully burned out or a prefired part. This can be achievedby isopressing using a flexible bag such as rubber or a conformalcoating. An alternative process would be to use clad hot isostaticpressing.

Conventional sintering of cold isopressed parts.

This invention for the first time, provides a complete process routingfor fabrication of a large cross section injection molded siliconnitride part which is crack free.

Table I shows the detailed process routing which has successfullyproduced crack free, internal as well as external, turbocharger sizedtest parts having cross sectional dimension up to 1.9 cm. Since, at thepresent time, silicon nitride powder is not available in the desiredparticle sizes, conventionally processed powder Si₃ N₄ +6 w/o Y₃ O₃ +2w/o Al₂ O₃, designated here as AY6, was calcined and remilled to obtainthe desired mean particle size.

                  TABLE I                                                         ______________________________________                                        Injection Molding Process Routing For A Large                                 Cross-Section Part.                                                           ______________________________________                                         ##STR1##                                                                      ##STR2##                                                                      ##STR3##                                                                     ______________________________________                                    

To obtain a ceramic powder such as AY6, having a desired mean particlesize of about 2 to about 12 microns, preferably from about 5 to about 10microns from a conventionally processed ceramic powder having a meanparticle size less than one micron, the conventionally processed ceramicpowder is calcined followed by comminuting to obtain the desiredparticle size. In the case of AY6, the calcining temperature was fromabout 1400° C. to about 1800° C. followed by milling for about 6 toabout 36 hours as illustrated in Table II as powder numbers 2, 3 and 4.

Table II summarizes the surface area, mean and median particle size (asmeasured by x-ray sedigraph) and the weight fraction of less than onemicron particles in the powder of the calcined and milled AY6 powders,powder numbers 2, 3 and 4, compared to a control AY6 powder which wasnot calcined. As Table II indicates, the calcining followed by millingof AY6 powder produces the desired mean particle sizes.

The calcined and milled AY6 ceramic powder is compounded with about 34v/o to about 42 v/o, preferably from about 37 v/o to about 40 v/o of awax based binder such as 90 w/o paraffin wax (Astor Chemical 1865Q), 5w/o of surfactant (Fisher oleic acid A-215), and 5 w/o of epoxythermosetting material (Acme 5144). The compounding is done in a Bramleytwo bladed dispersion mixer. The mixing chamber is heated to 80° C.Mixing is continued until the material has a creamy, homogenousappearance.

About 2 hours of mixing time subsequent to the initial blending of theparticulate and binder materials is sufficient. At this point a vacuumis applied and the mixing continued approximately 45 minutes to removeany entrapped air. The resulting mixture has rheological propertiescomparable to a thermoplastic material with a softening range of 40° to75° C. It can be pelletized or granulated according to well knowntechniques to a uniform particle size suitable as a feed material forinjection molding apparatus.

The molding is accomplished by known injection molding techniques.Injection molding is usually carried out utilizing the transfer methodor the direct injection method. In the transfer method a hydraulic pressforces the material from a heated storage chamber, by means of aplunger, through sprues or runners, into a mold. In the direct injectionmethod, the heated mixture is forced directly into the mold, throughrunners and gates, by either a hydraulic plunger or by reciprocatingscrew equipment. Either method may be utilized. The compounded materialwas injection molded into turbocharger sized shapes having crosssections up to 1.9 cm utilizing a 30 ton Trubor injection moldingmachine. Granulated material was loaded into the storage chamber andpreheated to the molding temperature. Optimum molding temperature isusually just above the melting point of the binder composition. Whereparaffin wax is the major binder component and epoxy is a minorcomponent, the chamber temperature was 70°-72° C. The die was maintainedat room temperature (24° C.). Molding pressure must be sufficient toforce the preheated mixture into all areas of the die. A pressure of3,000 to 10,000 psi is adequate for these materials, die and moldingconditions. The shot was injected into the die cavity and the pressureheld for 1/2 minute. The pressure was released, the die opened, and theparts removed from the die.

The injection molded green turbocharger sized parts were placed in atray and embedded in a setter powder of calcined AY6 powder having asurface area (BET) of 0.2 m² /g.

The binder was removed from the molded parts by heating the embeddedparts in a non-oxidizing environment such as nitrogen up to atemperature of 450° C. to completely remove the binder. During initialheating at 10° C./hr or greater in which 15 w/o to 20 w/o of the binderwas removed the setter powder formed a thick cake around the part. Thecake prevented further binder loss until the temperature wassufficiently high, approximately 400° C. up to 450° C., to break downthe barrier by the thermal decomposition and vaporization of the binder.Thus, the majority of the binder loss occurred after a temperature of400° C. was obtained and up to 450° C. The temperature of 450° C. wasthen raised to 600° C. and the heating was continued at 600° C. for upto 20 hours in air to remove the residual binder or carbon from thepart. For turbocharger sized test parts about 3 days of thermaltreatment was sufficient to completely remove the binder. For largerthan turbocharger sized cross section parts, a substantially lowerheating rate, as low as 1° C./hr may be required or a total thermaltreatment of approximately 17 days. The part was then cooled to roomtemperature. The resulting turbocharger sized part was free of externalcracks.

Other low surface area powder stable up to 600° C. could behave similarto silicon nitride setter powder and thus may be equally effective forburnout, binder removal, purposes. The use of a calcined and milledpowder (as specified in Table II) and a silicon nitride setter powdersuccessfully eliminated external cracks from more than 30 turbochargersized test parts having cross sections up to 1.9 cm.

                  TABLE II                                                        ______________________________________                                        INJECTION MOLDING SILICON NITRIDE POWDER                                      CHARACTERISTICS                                                                              X-ray Sedigraph                                                               Particle                                                       Pow-                         Mean  Median                                     der                          Size  Size  % less                               Num-  Processing     BET     (mi-  (mi-  than 1                               ber   Condition      (m.sup.2 /g)                                                                          cron) cron) micron                               ______________________________________                                        1.    Standard Processed                                                                           11.06   1.78   .88  65.0                                       AY6*                                                                    2.    Calcined. 1400° C. for                                                                5.00    5.61  2.06  31.0                                       4 h. milling for 6 h                                                    3.    Calcined. 1650° C. for                                                                4.62    5.51  3.94  19.5                                       4 h. milling for 36 h,                                                        classified                                                              4.    Calcined. 1800° C. for                                                                3.36    11.54 6.34  25.0                                       15 mins. milling for                                                          36 h                                                                    ______________________________________                                         *Data represents average of 5 different powder lots                      

The binder-free turbocharger sized part was then prefired to atemperature up to 1400° C., cooled to room temperature, and coldisostatically pressed, isopressed, in a flexible rubber bag frompressures of 5,000 psi up to 50,000 psi. However, the pressures greaterthan 50,000 psi can be used. To prevent sticking of the rubber bag tothe part a boron nitride lubricating layer can be used. The alternativeprocess is to isopress the binder-free part (which is generally fragileand requires careful handling) without prefiring. However, prefiring isrecommended for a complex part because it provides the strengthnecessary for handling and isopressing. Instead of using rubber bagswhich may be cumbersome for a complex cross section part (e.g.automotive gas turbine rotor), thin elastomer conformal coatings (e.g.commercially available plastisol, PVA, acrylics, or waxes) having athickness equal to or greater than 0.20 mm could be used. The thincoatings can be applied by various methods such as dipping, spraying,brushing, etc. After the coated part is isostatically pressed andremoved from the press, the thin coating is removed prior to thesintering step. The thin coating has been removed successfully by a burnoff cycle (to 550° C.) as well as removal by peeling. The isopressedpart can then be sintered in a conventional manner to provide 99% oftheoretical density. Table III gives some examples of crack-freeturbocharger sized samples fabricated via this process routing. Cracks,internal as well as external, could not be eliminated without the use ofan appropriate starting material, a proper setter and an isopressingoperation.

                  TABLE III                                                       ______________________________________                                        Examples of injection molded and sintered                                     turbocharger (cross section up to 1.9 cm)                                     sized test samples.                                                                  Prefiring            Part                                              Specimen                                                                             Temp and             Density                                           No.*   Time        Isopress (psi)  Comments                                   ______________________________________                                        46     --            5000   99.22  Visually 10×                                                            crack-free.                                48     --          10,000   99.21  Visually 10×                                                            crack-free.                                51     --          20,000   99.50  Visually 10×                                                            crack-free.                                52     1400° C.-4 h                                                                       25,000   99.00  Visually 10×                                                            crack-free.                                45     1000° C.-4 h                                                                       18,000   99.30  Visually 10×                                                            crack-free.                                ______________________________________                                         *Starting powder for all these specimens was powder no. 4 of Table II. Fo     binder removal, a calcined silicon nitride setter powder and a 10°     C./h heating in N.sub.2 to 450° C. followed by 20 hour hold at         600° C. in air was used.                                          

In the process routing, as shown in Table I, four areas are consideredcritical for fabrication of large, complex shapes: starting powder,binder removal setter powder, prefiring, and isopressing. The conceptsthat are used in the present invention are believed to be new and novel.The concept of isopressing a molded part which has binder removed(burnout) (with or without prefiring) can be used to improve thereliability of all ceramics of any shape and size and prepared bymethods other than injection molding (e.g. slip casting).

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A method for fabricating a large cross-sectionalceramic article by injection molding comprising the followingsteps:Step
 1. compounding a ceramic powder with about 34 to about 42 v/oof a binder to form a blend, said ceramic powder having a mean particlesize from about 2 to about 12 microns; Step
 2. injection molding theproduct from step 1 to form a molded ceramic article having across-section greater than about one centimeter; Step
 3. embedding theproduct from step 2 in a setter bed containing a setter powder to form abinder retarding layer of said setter powder on said molded ceramicarticle; Step
 4. heating the product of step 3 in a non-oxidizingenvironment at a heating rate equal to or greater than 1° C. per hour;Step
 5. retarding the removal of said binder from said molded ceramicarticle sufficiently to maintain equal to or greater than 80 w/o of saidbinder within said molded ceramic article until a temperature of 400° C.is obtained; Step
 6. increasing the temperature from step 5 to 450° C.at a heating rate equal to or greater than 1° C. per hour in anon-oxidizing environment to allow breakdown of the binder retardinglayer and to allow binder vaporization; Step
 7. increasing thetemperature from step 6 to about 600° C. and maintaining the product at600° C. in air for a period sufficient to completely remove said binderfrom said molded ceramic article; Step
 8. cooling the product from step7 to room temperature to obtain an externally crack-free injectionmolded and binder free ceramic article having a cross section greaterthan one centimeter; Step
 9. isopressing the product from step 8 at apressure equal to or greater than 5,000 psi; and Step
 10. sintering theproduct from step 9 at a temperature sufficient to obtain a densitygreater than 98% of theoretical density.
 2. A method for fabricating alarge cross-sectional ceramic article by injection molding comprisingthe following steps:Step
 1. compounding a ceramic powder with about 34to about 42 v/o of a binder to form a blend, said ceramic powder havinga mean particle size from about 2 to about 12 microns; Step
 2. injectionmolding the product from step 1 to form a molded ceramic article havinga cross-section greater than about one centimeter; Step
 3. embedding theproduct from step 2 in a setter bed containing a setter powder to form abinder retarding layer of said setter powder on said molded ceramicarticle; Step
 4. heating the product of step 3 in a non-oxidizingenvironment at a heating rate equal to or greater than 1° C. per hour;Step
 5. retarding the removal of said binder from said molded ceramicarticle sufficiently to maintain equal to or greater than 80 w/o of saidbinder within said molded ceramic article until a temperature of 400° C.is obtained; Step
 6. increasing the temperature from step 5 to 450° C.at a heating rate equal to or greater than 1° C. per hour in anon-oxidizing environment to allow breakdown of the binder retardinglayer and to allow binder vaporization; Step
 7. increasing thetemperature from step 6 to about 600° C. and maintaining the product at600° C. in air for a period sufficient to completely remove said binderfrom said molded ceramic articles; Step
 8. cooling the product from step7 to room temperature to obtain an externally crack-free injectionmolded and binder free ceramic article having a cross section greaterthan one centimeter; Step
 9. heating the product of step 8 to atemperature less than or equal to about 1500° C. in a nitrogenatmosphere; Step
 10. isopressing the product from step 9 at a pressureequal to or greater than 5,000 psi; and Step
 11. sintering the productfrom step 10 at a temperature sufficient to obtain a density greaterthan 98% of theoretical density.
 3. A method in accordance with claim 1wherein said ceramic powder comprises a silicon nitride powdercontaining sintering aids.
 4. A method in accordance with claim 3wherein said sintering aids are selected from the group consisting of Y₂O₃, Al₂ O₃, MgO and combinations thereof.
 5. A method in accordance withclaim 1 wherein said binder comprises about 37 v/o to about 40 v/o ofsaid blend.
 6. A method in accordance with claim 1 wherein said meanparticle size of said ceramic powder comprises from about 5 to about 10microns.
 7. A method in accordance with claim 1 wherein said crosssection of said molded ceramic article is greater than one centimeter upto 1.9 centimeter.
 8. A method in accordance with claim 1 wherein saidsetter powder comprises a silicon nitride powder containing 6 w/o Y₂ O₃and 2 w/o Al₂ O₃ and having a BET of 0.2 m² /g.
 9. A method inaccordance with claim 1 wherein said period sufficient to completelyremove said binder from within said molded ceramic article in step 5 isfrom about 3 to about 17 days.
 10. A method of fabricating an externallycrack free injection molded ceramic part having a large cross-sectioncomprising the following steps:Step
 1. calcining a ceramic powder havinga mean particle size less than one micron, said calcining beingsufficient to increase said mean particle size to greater than 2microns; Step
 2. comminuting the product from step 1 sufficiently toobtain a ceramic powder having a size from about 2 to about 12 microns;Step
 3. compounding the product from step 2 with about 34 to about 42v/o of a binder to form a blend, said ceramic powder having a meanparticle size from about 2 to about 12 microns; Step
 4. injectionmolding the product from step 3 to form a molded ceramic article havinga cross-section greater than about one centimeter; Step
 5. embedding theproduct from step 4 in a setter bed containing a setter powder to form abinder retarding layer of said setter powder on said molded ceramicarticle; Step
 6. heating the product of step 5 in a non-oxidizingenvironment at a heating rate equal to or greater than 10° C. per hour;Step
 7. retarding the removal of said binder from said molded ceramicarticle sufficiently to maintain equal to or greater than 80 w/o of saidbinder within said molded ceramic article until a temperature of 400° C.is obtained; Step
 8. increasing the temperature from step 7 to 450° C.at a heating rate equal to or greater than 1° C. per hour in anon-oxidizing environment to allow breakdown of the binder retardinglayer and to allow binder vaporization; Step
 9. increasing temperaturefrom step 8 to about 600° C. and maintaining the product at 600° C. inair for a period sufficient to completely remove said binder from saidmolded ceramic article; Step
 10. cooling the product from step 9 to roomtemperature to obtain an externally crack-free injection molded andbinder free ceramic article having a cross section greater than onecentimeter; Step
 11. heating the product of step 10 to a temperatureless than or equal to about 1500° C. in a nitrogen atmosphere; Step 12.isopressing the product from step 11 at a pressure equal to or greaterthan 5,000 psi; and Step
 13. sintering the product from step 12 at atemperature sufficient to obtain a density greater than 98% oftheoretical density.
 11. A method in accordance with claim 10 whereinsaid ceramic powder comprises a silicon nitride powder containingsintering aids.
 12. A method in accordance with claim 11 wherein saidsintering aids are selected from the group consisting of Y₂ O₃, Al₂ O₃,MgO and combinations thereof.
 13. A method in accordance with claim 10wherein said binder comprises about 37 v/o to about 40 v/o of saidblend.
 14. A method in accordance with claim 10 wherein said meanparticle size of said ceramic powder comprises from about 5 to about 10microns.
 15. A method in accordance with claim 10 wherein said setterpowder comprises a silicon nitride powder containing 6 w/o Y₂ O₃ and 2w/o Al₂ O₃ and having a BET of 0.2 m² /g.
 16. A method in accordancewith claim 10 wherein said period sufficient to completely remove saidbinder from within said molded ceramic article in step 5 is from about 3to about 17 days.
 17. A method in accordance with claim 10 wherein saidisopressing the product from step 10 at a pressure equal to or greaterthan 5,000 psi comprises coating the product from step 10 after coolingto room temperature with a thin elastomer conformal coating, coldisostatic pressing at a pressure equal to or greater than 5,000 psi andmaintaining that pressure for about 30 seconds, removing the coldisostatic pressed product from the press, and removing the thinelastomer conformal coating from the product.
 18. A method in accordancewith claim 17 wherein said thin elastomer conformal coating is equal toor greater than 0.2 mm thick.
 19. A method in accordance with claim 1wherein said binder comprises 90 w/o paraffin wax, 5 w/o surfactant, and5 w/o epoxy thermosetting material.
 20. A method in accordance withclaim 2 wherein said binder comprises 90 w/o paraffin wax, 5 w/osurfactant, and 5 w/o epoxy thermosetting material.
 21. A method inaccordance with claim 10 wherein said binder comprises 90 w/o paraffinwax, 5 w/o surfactant, and 5 w/o epoxy thermosetting material.
 22. Amethod of fabricating a large cross-sectional ceramic article byinjection molding comprising the following steps:Step 1--reducing theamount of ceramic particles having a size equal to or less than onemicron in a ceramic powder; Step 2--compounding said ceramic powder fromstep 1 with about 34 to about 42 v/o of a binder to form a blend; Step3--injection molding the product from step 2 to form a molded ceramicarticle having a cross-section greater than about one centimeter; Step4--embedding the product from step 3 in a setter bed containing a setterpowder to form a binder retarding layer of said setter powder on saidmolded ceramic article; Step 5--heating the product of step 4 in anon-oxidizing environment at a heating rate equal to or greater than 1°C. per hour; Step 6--retarding the removal of said binder from saidmolded ceramic article sufficiently to maintain equal to or greater than80 w/o of said binder within said molded ceramic article until atemperature of 400° C. is obtained; Step 7--increasing the temperaturefrom step 6 to 450° C. at a heating rate equal to or greater than 1° C.per hour in a non-oxidizing environment to allow breakdown of the binderretarding layer and allow binder vaporization; Step 8--increasing thetemperature from step 7 to about 600° C. and maintaining the product at600° C. in air for a period sufficient to completely remove said binderfrom said molded ceramic article; Step 9--cooling the product from step8 to room temperature to obtain an externally crack-free injectionmolded and binder free ceramic article having a cross section greaterthan one centimeter; Step 10--heating the produce of step 9 to atemperature less than or equal to about 1500° C. in a nitrogenatmosphere; Step 11--isopressing the product from step 10 at a pressureequal to or greater than 5,000 psi; and Step 12--sintering the productfrom step 11 at a temperature sufficient to obtain a density greaterthan 98% of theoretical density.
 23. A method in accordance with claim22 wherein said reducing of step 1 comprises calcining a ceramic powderhaving particles equal to or less than one micron at a temperature andfor a period sufficient to reduce the amount of said particles forming aceramic powder having a reduced amount of particles equal to or lessthan one micron; and comminuting said ceramic powder having a reducedamount of partices equal to or less than one micron for a periodsufficient to obtain a ceramic powder having a mean particle size fromabout 2 to about 12 microns.
 24. A method in accordance with claim 22wherein said ceramic powder comprises a silicon nitride powdercontaining sintering aids.
 25. A method in accordance with claim 23wherein said sintering aids are selected from the group consisting of Y₂O₃, Al₂ O₃, MgO and combinations thereof.
 26. A method in accordancewith claim 19 wherein said binder is about 37 v/o to about 40 v/o ofsaid blend.
 27. A method in accordance with claim 22 wherein said meanparticle size of said ceramic powder comprises from about 5 to about 10microns.
 28. A method in accordance with claim 22 wherein said setterpowder comprises a silicon nitride powder containing 6 w/o Y₂ O₃ and 2w/o Al₂ O₃ and having a BET of 0.2m² /g.
 29. A method in accordance withclaim 22 wherein said period sufficient to completely remove said binderfrom within said molded ceramic article in step 6 is from about 3 toabout 17 days.
 30. A method in accordance with claim 22 wherein saidbinder comprises 90 w/o paraffin wax, 5 w/o surfactant, and 5 w/o epoxythermosetting material.